Method of making a cathode-ray tube

ABSTRACT

Discoloration of easily reducible oxide containing glass in window portion of cathode-ray tube due to high energy electron bombardment is prevented by providing a thin transparent layer substantially free of easily reducible metal ions between the glass and the superimposed luminescent screen.

I United States Patent [191 [111 3,725,710

De Gier et al. [4 1 Apr. 3, 1973 [54] METHOD OF MAKING A CATHODE- [51] Int. Cl ..H0lj 29/00, HOlj 29/22 RAY TUBE [58] Field of Search ..l l7/33.5; 313/92 R [75] Inventors: Johannes De Gier; Reinhart Charles Willem Eisses, both of Emmasingel, [56] References Cited Eindhoven, Netherlands UNITED STATES PATENTS [73] Assignee: U.S. Philips Corporation, New 3,573,955 4/1971 De Gier et al ..i17/33.5 York, N.Y. 3,483,416 12/1969 Vermeulen et al. ..313/92 R [22] Filed: 1971 Primary Examiner-Herinan Karl Saalbach 211 App] 11 ,7 1 Att0meyFrank R. Trifari Related U.S. Application Data [57] ABSTRACT [62] Division of Ser. No. 542,967, April 7, 1965, Pat. No. Discoloration of easily reducible oxide containing 3,573,955 glass in window portion of cathode-ray tubedue to high energy electron bombardment is prevented by [30] Foreign Application Priority Data providing a thin transparent layer substantially free of easily reducible metal ions between the glass and the Apr. 17, 1965 Netherlands ..6S04936 Superimposed luminescent Screen [52] U.S. Cl. ..313/64, 313/92 R 6 Claims, 1 Drawing Figure PATENTEUAPR3 197a INVENTORS J. DE GIER, RglgHART CHARLES WILLEM EISSES AGENT METHOD OF MAKING A CATHODE-RAY TUBE This application is a division of U.S. Pat. application Ser. No. 542,967 filed Apr. 7, l965 and now U.S. Pat. No. 3,573,955.

The invention relates to a cathode-ray tube. In particular, it relates to a cathode-ray tube which is operated with comparatively high voltages, for example, for reproducing color television images.

Glass used for the screen of a cathode-ray tube must meet special requirements as regards its composition in order to avoid discoloration under the influence of the electron bombardment. For this purpose, cerium-containing glasses are used containing a maximum of l percent by weight of readily reducible compounds, such as oxides of lead, antimony and arsenic, and in addition a maximum of percent by weight of sodium oxide.

When electrons impinge upon the screen of a cathode-ray tube at a high speed, certain atoms of the luminescent substance which is provided on the inner surface of the screen, or even certain atoms of the glass may be excited so that these atoms emit x-ray radiation. With the voltages so far used in direct viewing cathoderay tubes for black and white reproduction this presents no difficulties. The energy of the electron impinging upon the screen is such that the statistical average is insufficient to excite atoms into producing xradiation; however, a small fraction are excited and do produce x-rays. The x-ray radiation produced, however, is of low intensity and the glass window, which is thick, absorbs most of it.

In the reproduction of color television images and in some projection systems, for example, it is necessary to use considerably higher voltages. Thus the intensity of the emerging x-ray radiation can be a serious problem. One solution might be to choose a type of glass for the window in which a considerable percentage of heavy atoms is present, particularly lead. However, under the influence of x-ray radiation glass also discolors with time. This can be prevented by choosing a type of glass which contains a few tenths to 1 percent of cerium. Such a type of glass which contains, for example, 10 percent of PbO and cerium to prevent discoloration caused by x-ray radiation, however, discolors appreciably when it is subjected to an intensive electron bombardment as the window of a cathode-ray tube for television reproduction as was to be expected on the basis of what was known already.

it is an object of this invention to provide a window for a cathode-ray tube suitable for reproducing color images which does not discolor or emit a significant amount of x-radiation. These and further objects of this invention will appear as the specification progresses.

According to the invention, a cathode-ray tube, particularly a tube suitable for the reproduction of color television images, is provided with a glass envelope which consists of an x-ray absorbent glass containing lead oxide or other readily reducible metal oxides, and which at least in the window and its vicinity comprises on its inner side between the luminescent layer and the glass a substantially transparent layer which contains a maximum of 1 percent of reducible metal ions.

If desired, a layer of a substance which causes an even absorption of the luminescent light in such manner that the contrast is still acceptable in the case of bright ambient illumination may also be provided.

In a preferred embodiment, the layer containing a maximum of 1 percent of reducible metal ions only has to have a thickness of one-fourth to 10 microns. It is particularly surprising that such a thin layer is capable of completely absorbing the electrons passed by the layer of the luminescent substance or at least making them harmless in such manner that no appreciable discoloration occurs in the glass of the window by reduction of lead oxide or of other readily reducible metal oxides.

In one embodiment this layer may consist of vitreous SiO which can be provided in a simple manner, for example, by treating the inner side of the window,prior to applying the luminescent layer, with a solution of. ethyl silicate which may be partly hydrolized, followed by hydrolysis, drying, and heating at a temperature of, for example, 350 C.

A similar layer can be obtained by treating the window with a solution of water glass and then heating the window. Alternatively, the layer may be obtained by flocculation of a silicate solution, for example, by means of some multivalent positive ions. If required, the positive ions may be absorbed on the glass surface before the treatment with the silicate solution.

In another embodiment the layer is provided by means of a suspension, for example, of glass of the type which is known to show no discoloration under electron bombardment, as described above, or of anon-activated luminescent substance. I

In still another embodiment a cathode-ray tube is provided with a layer which receives the impinging electrons so that they cannot cause discoloration in the lead-containing glass of the envelope. In this embodiment the latter layer is manufactured from a surface layer of the glass itself by leaching PbO, and, if required, other readily reducible metal oxides and/or Na O, at least for a predominant part. The Na O is of importance in this connection because the Na+ ion, as a result of its great mobility, contributes to the reduction of the heavy metal oxide. This leaching is carried out by exposing the glass for some time to a melt which contains potassium ions at a temperature between 350 and 450 C, as a result of which a substitution of potassium ions for sodium ions takes place.

The invention will now be described with reference to the accompanying drawing in which the sole figure shows the window portion of a cathode-ray tube with the luminescent layer thereon and with reference to the following illustrative example.

A cathode-ray tube according to the invention comprises a glass window 1 which is sealed to a cone 2 in the usual manner. Prior to depositing the luminescent layer 4, a thin transparent layer 3 is provided between the luminescent layer and the window which, as described hereinafter, serves to prevent discoloration of the window under electron bombardment.

A bulb was manufactured for a cathode-ray tube of the type designed for projection of television images, the glass of the window having the following composition in percent by weight:

MnO

Fe o 0.053

Remainder (impurities) 0.33

The glass of the bulb was tempered by heating in the normal manner at 440 C. Thereafter, the bulb was washed with a warm 6 percent solution of soda and then rinsed with demineralized water. Half of the window was coated with a readily removable lacquer consisting of polyvinylchloroacetate (the line of separation coincides with the diameter of the circular screen). A 7 percent solution of potassium silicate was then poured into the bulb and, after the whole screen had been wetted by the solution, poured out again. The bulb was then heated at 120 C, so that the lacquer was dried and the silicate layer was dried in; the lacquer layer could then be removed easily together with the layer of potassium silicate deposited on it. On the part where no lacquer had been provided, the potassium silicate adhered to the glass. The neck of the bulb was rinsed with a 6 percent hydrofluoric acid solution. The normal Allayer was provided on the window by vapor-deposition. The luminescent layer was omitted because this was superfluous for illustrating the effect according to the invention. The normal gettering material was atomized (barium-nickel), the bulb was provided with an electron gun, evacuated at 380 C and sealed.

The tube thus obtained was provided with deflection coils and operated with a cathode voltage of kV for 35 hours, a surface of 78 cm being scanned by the nonmodulated electron beam. The screen load was 200/;LA.

It was then found that the part of the screen which had no potassium silicate layer showed an intensive brown coloration while the part which was coated with a potassium silicate layer, showed substantially no discoloration.

While the invention has been described with reference to particular embodiments thereof, other modifications will be apparent to those skilled in this art without department from the scope of the invention as defined in the appended claims.

We claim:

1. A cathode-ray tube comprising a glass envelope having a window portion which consists of a glass capable of absorbing x-rays, said glass containing at least one easily-reducible metal oxide and capable of becoming discolored upon bombardment by high energy electrons, means in said tube for producing a stream of high energy electrons and directing said stream at said window portion a phosphor layer on said window portion, and means positioned between said phosphor layer and said window portion for preventing said high energy electrons from discoloring said window portion, said discoloring prevention means comprising a solid thin substantially transparent layer covering the inner surface of said window portion and containing a maximum of 1 percent by weight of an easily reducible metal oxide.

2. The cathode-ray tube of claim 1 wherein the transparent layer has a thickness between one-fourth and 10 microns.

3. The cathode-ray tube of claim 1 wherein the transparent layer consists substantially of a layer of vitreous SiO 4. The cathode-ray tube of claim 1 wherein the transparent layer consists substantially of a layer of potassium silicate. t

5. The cathode-ray tube of cla m 1 wherein the win- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3J25,710 (PHN 827A) Dated April 3,1973

Inventor(s) JOHANNES DE GIER ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shownbelow:

q Item E62], "April 7 should be April 15 Signed and sealed this 1st day of January 197k.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Q I A'ttesting Officer Acting Commissioner of Patents 

2. The cathode-ray tube of claim 1 wherein the transparent layer has a thickness between one-fourth and 10 microns.
 3. The cathode-ray tube of claim 1 wherein the transparent layer consists substantially of a layer of vitreous SiO2.
 4. The cathode-ray tube of claim 1 wherein the transparent layer consists substantially of a layer of potassium silicate.
 5. The cathode-ray tube of claim 1 wherein the window portion contains lead and sodium ions and the transparent layer is a continuation of said window portion but wherein potassium ions are present in place of the lead and sodium ions.
 6. The cathode-ray tube of claim 1 wherein the window portion contains lead ions of given concentration and the said transparent layer consists of the glass of said window portion having a lead-ion concentration less than said given concentration. 